Change main.version from const to var

[kei2100]

WHAT

Change main.version from const to var.
If applied, this commit will fix #83

Here's the Github release that includes this commit.
https://github.com/kei2100/tfnotify/releases/tag/v0.7.1-beta.1

$ wget https://github.com/kei2100/tfnotify/releases/download/v0.7.1-beta.1/tfnotify_darwin_amd64.tar.gz

$ tar xfv tfnotify_darwin_amd64.tar.gz

$ ./tfnotify --version
tfnotify version 0.7.1-beta.1